One method for measuring time intervals is by directly counting the lapse of time in the interval. Such a method is described in Hewlett-Packard Application Note 172, "The Fundamentals of Electronic Frequency Counters," pp. 23 ff. In this method, the start event opens a gate to allow a reference clock to be counted until the stop event closes it. Since the maximum resolution of this method is one clock period, there is a .+-.1 count ambiguity. Therefore, methods to improve the resolution have been devised. One direct method is to decrease the significance of the .+-.1 ambiguity by increasing the clock frequency. This method, however, often requires more sophisticated and costly circuits for implementation. Furthermore, a typical commercial instrument using this method has only an approximately 1-2 nanosecond resolution at best.
For repetitive time intervals, the time interval can be repetitively measured and averaged. As a result, the resolution of the measurement is improved by the averaging due to the statistical nature of the .+-. count ambiguity. The resolution is improved by a factor of 1/.sqroot.A, where A is the number of intervals averaged. See, for instance, HP AN 172, pp. 30-31. If the reptition rate of the interval is coherent with the clock, however, there would be no difference in the results of the repeated measurements, and no improvement would be achieved by such an averaging. For such cases, special techniques must be used for averaging to take place. See David C. Chu, "Time Interval Averaging: Theory, Problems, and Solutions," Hewlett-Packard Journal, June 1974, pp. 12-15 and U.S. Pat. No. 3,886,451.
Several methods have been developed for interpolating between clock pulses to achieve higher resolution without averaging. One method, described in U.S. Pat. No. 3,133,189; Gilbert A. Reeser, "An Electronic Counter for the 1970's," Hewlett-Packard Journal, May 1969, pp. 9-12; and Ronald Nutt, "Digital Time Intervalometer," The Review of Scientific Instruments, Vol. 39, No. 9, Sept. 1968, pp. 1342-1345, involves charging a capacitor at a rapid rate and discharging it at a much slower rate. With a much slower rate, the discharge time can be measured directly. This technique is used to determine both the start and the stop ambiguities, and the two numbers thus derived are combined arithmetically with the direct count measurement to give the final result. A second interpolation method, described in U.S. Pat. No. 3,218,553 and by Patrick Young, "1 Nanosecond Time Interval Counter," Instruments & Control Systems, uses two startable oscillators slightly offset from each other in frequency to give a vernier interpolation. One oscillator output signal is started by the start signal so there would be no start ambiguity; it is counted directly. The second oscillator output signal of a slightly higher frequency is started by the stop signal and is counted until it becomes coincident with the first oscillator output signal, thus giving a vernier interpolation of the stop ambiguity. Both of these methods require the ability to combine arithmetically several numbers, a capability that does not normally exist without special circuitry. In addition, the implementation of these methods themselves may require sophisticated and specialized circuitry.
The interpolation methods discussed above have been used in generally available production instruments. There are also some methods that have been developed and used in specialized applications, e.g., for nuclear physics. Examples of these specialized methods include the technique of time to pulse height conversion; the chronotron technique, which uses incremental delay elements of slightly different lengths in the start and stop channels; and a technique that involves forming a pulse whose width is equal to the time interval. In the last method mentioned, the pulse formed is sent down a delay line and the output of the delay line is used to form another pulse. In this manner, the time interval is repeated. See the descriptions in William H. Venable, Jr., "Tunnel Diode Chronotron Circuit for Picosecond Range," The Review of Scientific Instruments, Vol. 37, No. 11, November 1966, pp. 1443-1449 and Gunther Franke, Roberto Pevararo and Heinz Fischer, "Measuring Nanosecond Time Differences by Dynamic Storage of Flip-Flop Pulses," Proceedings of the IEEE, Vol. 56, February 1968, pp. 221-222. This method, though, suffers from limitations such as a restricted operation range and degradation of the repeated interval due to unmatched delays. The present invention also involves reproducing the time interval to be measured. However, it does not suffer from these limitations.